Background/Question/Methods The opossum shrimp Mysis relicta is a major predator on zooplankton and both prey for and competitor with planktivorous fish in deep northern lakes. Mysids undergo diel vertical migration from their daytime benthic habitat into the pelagic realm to feed at night and their vertical distribution is strongly influenced by temperature and light. We have developed a model based on laboratory-derived light and temperature preference functions that was capable of predicting mysid vertical distributions on three different nights in Lake Ontario when temperature or light was the dominant environmental factor limiting vertical distribution (i.e., conditions with strong thermoclines or an isothermal water column). However, it is still unclear as to how temperature and light interact to influence mysid vertical distribution across the range of environmental conditions likely to be experienced by mysids in a given year. In addition, little is known about how other factors such as predator presence and prey densities may modify mysid vertical distribution.
Results/Conclusions We measured the vertical distribution of both mysid predators (planktivorous fish) and mysid prey (zooplankton) in addition to mysids, light and temperature on eight occasions from May to September in Lake Ontario that spanned different light and temperature regimes. We use this data to test two different predictive models of mysid habitat selection. The first is based on our laboratory-derived responses of mysids to different light and temperature gradients. The second is based on predator:prey density ratios and incorporates effects of light and temperature indirectly through their effect on predation rates. We show that the model based on laboratory derived preferences is a better predictor of mysid vertical distribution than the predator:prey density ratio. Predation risk may be too variable and too difficult for mysids to accurately assess to allow for development of direct responses to fish abundance. Instead, we believe mysids have developed genetically fixed preferences to light levels and temperature conditions that minimize fish feeding rate regardless of fish density.